Pressure switches are widely used in industrial applications relating to process fluids. They are used to regulate pump and compressor operation as well as liquid levels in tanks within specific predetermined pressure ranges. These pressure switches typically have two set points: a high (or tripping) pressure, and a low (or reset) pressure.
Oilfield pumps and vessels containing liquid are common applications for pressure switches. Oilfield pumps are set up with a pressure switch located in the discharge line to detect high and low pressures. When the pressure within the discharge line senses the tripping pressure, the switch triggers the pump to shut down. When the line pressure drops below the reset pressure, the switch will trigger the pump to resume operation.
Another common application for pressure switches is in association with a tank containing liquid, where a pump is required to fill or empty the vessel. A pressure switch allows the operator to set the pump to operate automatically within specific liquid head pressures.
As may be seen by way of example in U.S. Pat. Nos. 5,554,834 and 5,670,766, a conventional pressure switch typically features an electrical enclosure at its upper end and a spring body and process connection at its lower end, which is mountable to an opening in a pipeline, vessel, or other component containing a fluid. A metal push rod is slidably mounted within the spring body housing, with its lower end operatively engaged with a metal piston and metal diaphragm assembly which closes off the process connection, such that the diaphragm will be exposed to pressure from the pipeline or vessel. The upper end of the push rod extends into the electrical enclosure. An electrical microswitch is disposed within the electrical enclosure and securely fastened to a mounting bracket. The microswitch has conventional contacts for wiring to whatever electrically-actuated device the pressure switch, is intended to control. The microswitch also has, on its lower side, a plunger or trigger which when pressed into the microswitch (i.e., upward relative to the enclosure) will trip the microswitch.
The assembly described above is configured such that upward movement of the piston is transferred to the push rod in response to external fluid pressure applied to the diaphragm, such that the push rod trips the microswitch. The specific mechanism used to translate push rod movement into trigger movement may vary from one manufacturer to the next.
A conventional pressure switch typically incorporates a spring assembly including a helical spring of suitable stiffness, disposed around the push rod and extending between the diaphragm end of the push rod and an upper abutment within the enclosure. This spring assembly provides a resistive force necessary to maintain a specific range of pressure to both trip and reset the device. Accordingly, a higher tripping pressure will entail a higher degree of spring, compression. To facilitate adjustment of the spring compression, the aforementioned abutment is longitudinally movable within the pressure switch housing.
One of the critical challenges in the design of pressure switches is to provide for accurate and reliable pre-setting of desired tripping and reset pressures, which essentially boils down to finely controlled adjustment of the gap between the microswitch and push rod assembly.
In some pressure switch designs, the microswitch could be tripped by effectively direct actuation of the trigger by the upper end of the push rod; in such designs, however, accurate control of the gap between the push rod and microswitch trip button would be difficult, since it would require the components to be machined to unrealistic tolerances.
In other designs, a deformable offset trip plate (typically made of steel) may be provided in association with the microswitch such that the trigger is laterally offset from the axis of the push rod, but upward movement of the push rod will raise the free end of the trip plate, in turn causing another portion of the trip plate to exert an upward force on the trigger. However, this involves a tedious and time-consuming trial-and-error procedure. With the switch partially disassembled, the trip plate must be bent into a trial position, whereupon the switch is reassembled and connected to an external pressure source to determine the actual tripping pressure that corresponds to the trip plate position. If the gap between the push rod and the microswitch is too wide or too narrow, the switch must be disassembled again so that the trip plate can be bent one way or the other into a new trial position, and then the switch is reassembled and tested again. This procedure is followed until the trip plate is in a position that produces the appropriate gap between the push rod and the microswitch.
For the foregoing reasons, there is a need for a pressure switch calibration mechanism that facilitates fine adjustment of the gap between the push rod and microswitch more easily and more quickly than is possible with typical conventional switches, and without need for trial-and-error methods. The present invention is directed to this need.